Crystal resonant frequency sensor

Abstract

A method and apparatus for measuring the frequency of a desired resonant mode of a crystal arrangement, or other two-port device, during an automated operation. The crystal arrangement, or other two-port device, is placed into a test circuit and subjected to a sinusoidal test signal of known frequency. Based upon the output response of the crystal arrangement to the test signal, the frequency of the test signal is changed such that the test signal rapidly converges on a desired mode of operation of the crystal arrangement. This is accomplished by first noting a desired increase in amplitude of the output response of the crystal arrangement, followed by measuring an error signal related to the desired crystal arrangement mode of operation. When the error equals a predetermined value the frequency of the sinusoidal test signal is the frequency of the desired mode.

What is claimed is:  
     
       1. A sensor for measuring the frequency of a desired resonant mode of a crystal arrangement, comprising: 
       a voltage-controlled oscillator, having an input and an output, for generating at said oscillator output a signal whose frequency is responsive to a command signal at said input of said oscillator; and  
       a phase shifter having an input and an output, said input of said phase shifter electrically coupled to said output of said voltage-controlled oscillator, said crystal arrangement electrically connected in shunt with said output of said phase shifter;  
       a first multiplier circuit having two inputs, both being coupled with the output of the phase shifter wherein a passive response of said crystal arrangement is multiplied with itself by said first multiplier circuit to generate a first signal having a characteristic amplitude versus frequency curve which peaks substantially at the frequency of the desired resonant mode of the crystal arrangement;  
       a second multiplier circuit having two inputs, one being coupled with the output of the voltage-controlled oscillator and the other being coupled with the output of the phase shifter wherein the two inputs are multiplied by said second multiplier circuit to generate a second signal having a characteristic amplitude versus frequency curve which crosses a predetermined value substantially at the frequency of the desired resonant mode of the crystal arrangement, said crossing occurring within a frequency range in which the first signal is above a predetermined threshold value; and  
       a controller for generating said command signal based on values of said first and second signals;  
       wherein said voltage-controlled oscillator, said phase shifter, said first and second multiplier circuits and said controller form a control loop, wherein said controller adjusts said command signal such that said first signal approaches the predetermined threshold value and said second signal approaches the predetermined value, wherein when said first signal exceeds the predetermined threshold value and said second signal equals substantially the predetermined value the frequency of said output signal of said voltage-controlled oscillator is correlated with the frequency of said desired resonant mode of said crystal arrangement.  
     
     
       2. A sensor as in claim  1 , wherein said predetermined value is zero. 
     
     
       3. A sensor as in claim  1 , wherein said phase shifter is comprised of a capacitor. 
     
     
       4. A sensor as in claim  1  further comprising: 
       a first filter circuit electrically disposed between said output of said first multiplier and said controller, said first filter circuit being responsive to said first signal for providing a filtered first signal to said controller; and  
       a second filter circuit electrically disposed between said output of said second multiplier and said controller, said second filter circuit being responsive to said second signal for providing a filtered second signal to said controller.  
     
     
       5. A sensor as in claim  1 , wherein said controller is programmable, and further comprising means for programming said programmable controller. 
     
     
       6. A sensor as in claim  5 , wherein said means for programming said programmable controller includes an input device for passing user inputted parameters to said programmable controller. 
     
     
       7. A sensor as in claim  1  wherein the controller comprises a microprocessor. 
     
     
       8. A sensor for measuring the resonant frequency of multiple, closely spaced modes of a crystal arrangement, comprising: 
       a voltage-controlled oscillator having an input and an output, said oscillator being adapted to provide an oscillating output signal whose frequency is responsive to a command signal at said input of said oscillator;  
       a programmable phase shifter having an input and an output, wherein an error signal is generated which is proportional to a phase difference between said input of said programmable phase shifter and said output of said programmable phase shifter, said input of said programmable phase shifter electrically coupled to said output of said voltage-controlled oscillator, wherein said crystal arrangement being electrically connected in shunt with said output of said programmable phase shifter; means for programming said programmable phase shifter;  
       a first multiplier circuit having at least two inputs and one output, two of said inputs of said first multiplier being electrically coupled with said output of said programmable phase shifter, wherein a passive response of said crystal arrangement is amplified;  
       a second multiplier circuit having at least two inputs and one output, one of said inputs being electrically coupled with said output of said voltage-controlled oscillator, the second of said inputs being electrically coupled to said output of said programmable phase shifter;  
       a controller having at least two inputs and at least one output, wherein said output is electrically coupled to said input of said voltage-controlled oscillator for providing said command signal to said voltage-controlled oscillator, one of said inputs is electrically coupled to said output of said first multiplier, and a second of said inputs is electrically coupled to said output of said second multiplier;  
       a first filter circuit electrically dispose between said output of said first multiplier and said controller, said first filter circuit having an input and an output and being responsive to said output of said first multiplier applied to said input of said first filter circuit for providing a filtered output signal to said first input of said controller, said output of said first filter circuit defining a signal proportional to an amplitude response of said crystal arrangement to said oscillating output signal of said voltage-controlled oscillator; and  
       a second filter circuit electrically disposed between said output of said second multiplier and said controller, said second filter circuit having an input and an output and being responsive to said output of said second multiplier applied to said input of said second filter circuit for providing a filtered output signal to said second input of said controller, wherein said output of said second multiplier defining an error signal indicative of one of said multiple resonant modes of said crystal arrangement;  
       when in an open-loop mode, said voltage-controlled oscillator, said programmable phase shifter, said first multiplier, said first filter, and said controller form a control loop, wherein said output of said programmable phase shifter is adjusted until said output exceeds a predetermined threshold value;  
       when in a closed-loop mode, said voltage-controlled oscillator, said programmable shifter, said second multiplier, said second filter, and said controller form the control loop, wherein said output of said programmable phase shifter is adjusted until the value of said error signal equals a predetermined value, whereby the frequency of said oscillating output of said voltage-controlled oscillator is correlated with the frequency of one of said multiple modes of said crystal arrangement.  
     
     
       9. A method for determining the frequency of a desired resonant mode of a crystal arrangement, comprising the steps of: 
       calculating operating parameters related to the desired resonant mode of the crystal arrangement, the operating parameters comprising a frequency range of operation, a final error value, and a threshold value;  
       providing a control signal to a voltage-controlled oscillator, wherein the frequency of a sinusoidal output signal of the voltage-controlled oscillator is responsive to the control signal; and  
       providing the sinusoidal output signal of the voltage-controlled oscillator to the crystal arrangement  
       wherein the frequency of the sinusoidal signal being within the calculated frequency range, the sinusoidal output signal first passing through a phase shifter having an input and output, wherein an error signal is generated which is proportional to a phase difference between the input of the phase shifter and the output of the phase shifter,  
       when in an open-loop mode, measuring a response of the crystal arrangement to the sinusoidal output signal;  
       in the open-loop mode, adjusting the frequency of the sinusoidal output signal until the value of the measured response of the crystal arrangement is greater than the calculated threshold value;  
       when in a closed-loop mode, measuring the error signal; and  
       in the closed-loop mode, adjusting the frequency of the sinusoidal output signal until the value of the measured error signal is equal to the calculated final error signal value, whereby the frequency of the sinusoidal output signal is the frequency of the desired resonant mode of the crystal arrangement.  
     
     
       10. A method as in claim  9 , wherein the step of calculating operating parameters further comprises the step of calculating a system gain, wherein the rate at which the frequency of the sinusoidal signal can be adjusted is increased such that less time is required to determine the frequency of the desired mode of the crystal arrangement. 
     
     
       11. A method as in claim  9 , wherein the step of adjusting the frequency of the sinusoidal signal until the value of the measured response of the crystal arrangement is greater than the calculated threshold value further comprises the step of adjusting the control signal to the voltage-controlled oscillator, wherein the frequency of the sinusoidal output of the voltage-controlled oscillator sweeps within the calculated frequency range, and wherein the value of the response of the crystal arrangement approaches the calculated threshold value. 
     
     
       12. A method as in claim  11 , wherein the step of adjusting the frequency of the sinusoidal signal until the value of the measured error signal is equal to the calculated final error signal value further comprises the step of adjusting the control signal to the voltage-controlled oscillator wherein the frequency of the sinusoidal output of the voltage-controlled oscillator is finely adjusted within the calculated frequency range, and wherein the value of the response of the crystal arrangement approaches the calculated final error value. 
     
     
       13. A method for determining the resonant frequency of multiple, closely spaced modes of a crystal arrangement, comprising the steps of: 
       calculating operating parameters related to one of the multiple, closely spaced resonant modes of the crystal arrangement, the operating parameters comprising a frequency range of operation, a final error value, and a threshold value;  
       providing a control signal to a voltage-controlled oscillator, wherein the frequency of a sinusoidal output of the voltage-controlled oscillator is responsive to the control signal, and the frequency of the sinusoidal output of the voltage-controlled oscillator is within the calculated frequency range;  
       providing the sinusoidal output of the voltage-controlled oscillator to the crystal arrangement, the sinusoidal output first passing through a phase shifter having an input and an output, wherein an error signal is generated which is proportional to the phase difference of said input of said phase shifter and said output of said phase shifter;  
       when in an open-loop mode, measuring a response of the crystal arrangement to the sinusoidal output;  
       in the open-loop mode, adjusting the control signal to the voltage-controlled oscillator, wherein the frequency of the sinusoidal output of the voltage-controlled oscillator sweeps within the calculated frequency range, and wherein the value of the response of the crystal arrangement approaches and exceeds the calculated threshold value;  
       when in a closed-loop mode, measuring the error signal; and  
       in the closed-loop mode, adjusting the control signal to the voltage-controlled oscillator wherein the frequency of the sinusoidal output of the voltage-controlled oscillator is finely adjusted within the calculated frequency range until the value of the measured error signal is equal to the calculated final error signal value, whereby the frequency of the sinusoidal output is the frequency of one of the multiple, closely spaced resonant modes of the crystal arrangement.